Imaging device including unit pixel cell

ABSTRACT

An imaging device comprising a unit pixel cell comprising: a photoelectric converter that generates an electric signal through photoelectric conversion of incident light; and a signal detection circuit that detects the electric signal, the signal detection circuit comprising a first transistor that amplifies the electric signal, a second transistor that selectively transmits output of the first transistor to outside of the unit pixel cell, and a feedback circuit that forms a feedback loop through which the electric signal is negatively fed back, the feedback loop not passing through the first transistor.

BACKGROUND

1. Technical Field

The present disclosure relates to an imaging device.

2. Description of the Related Art

In recent years, imaging devices have been widely used in the fields ofvarious products, such as video cameras, digital still cameras, securitycameras, and vehicle-mounted cameras. The imaging devices areimplemented by charge-coupled device (CCD) solid-state imaging devicesor complementary metal-oxide semiconductor (CMOS) solid-state imagingdevices. Of the devices, the CMOS solid-state imaging devices are mainlyused. Since CMOS solid-state imaging devices can be manufactured using ageneral CMOS process, existing facilities can be used, and thus imagingdevices can be stably supplied. In addition, since peripheral circuitrycan be incorporated into the same chip, signals can be read from theimaging devices at high speed, thus making it possible to achieve ahigher speed and a higher resolution. Thus, CMOS solid-state imagingdevices have many advantages.

For example, correlated double sampling (CDS) like that disclosed inJapanese Unexamined Patent Application Publication No. 2010-129705(hereinafter referred to as “Patent Document 1”) is widely used in CMOSsolid-state imaging devices.

SUMMARY

In the field of imaging devices, noise reduction is required. Inparticular, reduction of kTC noise generated during resetting (alsocalled “reset noise”) is required.

One non-limiting and exemplary embodiment provides an imaging devicethat can effectively suppress reset noise.

In one general aspect, the techniques disclosed here feature an imagingdevice comprising a unit pixel cell comprising: a photoelectricconverter that generates an electric signal through photoelectricconversion of incident light; and a signal detection circuit thatdetects the electric signal, the signal detection circuit comprising afirst transistor that amplifies the electric signal, a second transistorthat selectively transmits output of the first transistor to outside ofthe unit pixel cell, and a feedback circuit that forms a feedback loopthrough which the electric signal is negatively fed back, the feedbackloop not passing through the first transistor.

It should be noted that general or specific embodiments may beimplemented as a system, a method, an integrated circuit, a computerprogram, a storage medium, or any selective combination thereof.Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a pixel circuit in a typicalimaging device;

FIG. 2 is a timing chart illustrating one example of the operation ofthe pixel circuit in the typical imaging device;

FIG. 3 is a schematic diagram illustrating an exemplary circuitconfiguration of an imaging device according to a first embodiment;

FIG. 4 is a schematic diagram illustrating an exemplary circuitconfiguration of each unit pixel cell in the imaging device according tothe first embodiment;

FIG. 5A is a schematic diagram illustrating an example of theconfiguration of a photodetector;

FIG. 5B is a schematic diagram illustrating an example of theconfiguration of the photodetector;

FIG. 6 is a schematic diagram illustrating an exemplary circuitconfiguration of a reading circuit according to the first embodiment;

FIG. 7 is a timing chart illustrating one example of the operation ofthe reading circuit;

FIG. 8 is a timing chart illustrating another example of the operationof the reading circuit;

FIG. 9 is a schematic diagram illustrating an exemplary circuitconfiguration of each unit pixel cell in an imaging device according toa second embodiment;

FIG. 10 is a schematic diagram illustrating an exemplary circuitconfiguration of a reading circuit according to the second embodiment;

FIG. 11 is a timing chart illustrating one example of the operation ofthe reading circuit;

FIG. 12 is a schematic diagram illustrating an exemplary circuitconfiguration of the reading circuit according to the second embodiment;

FIG. 13 is a schematic diagram illustrating another exemplary circuitconfiguration of the reading circuit according to the second embodiment;

FIG. 14 is a schematic diagram illustrating another exemplary circuitconfiguration of the reading circuit according to the second embodiment;

FIG. 15 is a schematic diagram illustrating another exemplary circuitconfiguration of the reading circuit according to the second embodiment;

FIG. 16 is a schematic diagram illustrating another exemplary circuitconfiguration of the reading circuit according to the second embodiment;

FIG. 17 is a timing chart illustrating another example of the operationof the reading circuit;

FIG. 18 is a schematic diagram illustrating an exemplary circuitconfiguration of a unit pixel cell in an imaging device according to athird embodiment;

FIG. 19A is a schematic diagram illustrating an exemplary circuitconfiguration of a reading circuit according to the third embodiment;

FIG. 19B is a schematic diagram illustrating another exemplary circuitconfiguration of the reading circuit according to the third embodiment;

FIG. 20 is a timing chart illustrating one example of the operation ofthe reading circuit;

FIG. 21A is a schematic diagram illustrating another exemplary circuitconfiguration of the reading circuit according to the third embodiment;

FIG. 21B is a schematic diagram illustrating another exemplary circuitconfiguration of the reading circuit according to the third embodiment;

FIG. 22A is a schematic diagram illustrating another exemplary circuitconfiguration of the reading circuit according to the third embodiment;

FIG. 22B is a schematic diagram illustrating another exemplary circuitconfiguration of the reading circuit according to the third embodiment;

FIG. 23 is a schematic diagram illustrating an exemplary circuitconfiguration of each unit pixel cell in an imaging device according toa fourth embodiment;

FIG. 24 is a schematic diagram illustrating an exemplary circuitconfiguration of a reading circuit according to the fourth embodiment;

FIG. 25 is a timing chart illustrating one example of the operation ofthe reading circuit;

FIG. 26 is a timing chart illustrating another example of the operationof the reading circuit;

FIG. 27 is a schematic diagram illustrating another exemplary circuitconfiguration of the reading circuit according to the fourth embodiment;

FIG. 28 is a schematic diagram illustrating an exemplary circuitconfiguration of each unit pixel cell in an imaging device according toa fifth embodiment;

FIG. 29 is a schematic diagram illustrating another exemplary circuitconfiguration of the unit pixel cell in the imaging device according tothe fifth embodiment;

FIG. 30 is a schematic diagram illustrating an exemplary circuitconfiguration of a reading circuit according to the fifth embodiment;

FIG. 31 is a timing chart illustrating one example of the operation ofthe reading circuit;

FIG. 32 is a timing chart illustrating another example of the operationof the reading circuit;

FIG. 33 is a schematic diagram illustrating another exemplary circuitconfiguration of the reading circuit according to the fifth embodiment;

FIG. 34 is a schematic diagram illustrating another circuitconfiguration of the reading circuit according to the fifth embodiment;and

FIG. 35 is a schematic diagram illustrating a camera system according toa sixth embodiment.

DETAILED DESCRIPTION

Before a description is given of embodiments of the present disclosure,typical CDS will first be described with reference to FIGS. 1 and 2.

FIG. 1 illustrates a typical pixel circuit. The pixel circuit includes aphotodetector (PD) portion, a floating diffusion (FD), a transfer (TX)transistor, a reset (RES) transistor, amplification (AMP) transistor,and a selection (SEL) transistor. The PD portion detects an opticalsignal. The FD converts charge generated in the PD portion into avoltage signal and temporarily holds the charge. The TX transistortransfers a charge signal from the PD portion to the FD. The REStransistor resets the voltage of the FD. The AMP transistor amplifiesthe voltage signal of the FD. The SEL transistor outputs the amplifiedsignal. The PD portion is typically a photodiode.

FIG. 2 is a timing chart illustrating one example of the operation ofthe pixel circuit disclosed in Patent Document 1. At time t1, an RESsignal is set to a low level, so that the RES transistor is turned off.At time t2, the SEL signal is set to a high level, so that the SELtransistor is turned on, and a reset voltage is output. When the SELtransistor is turned on, the reset voltage contains kTC noise, and thus,fluctuations over time occur in signals. Thereafter, at time t3, a TXsignal is set to a high level, so that the TX transistor is turned on.As a result, the signal charge accumulated in the PD portion istransferred to the FD. Next, at time t4, the TX signal is set to a lowlevel, so that the TX transistor is turned off. As a result, the signalvoltage is determined. The determined signal voltage is a signal thathas changed relative to the reset voltage by an amount corresponding toa voltage according to the accumulated signal charge. Thus, in a circuitat a subsequent stage, it is possible to detect only a voltagecorresponding to the accumulated signal charge and to thereby cancel thekTC noise, by obtaining a difference between the reset voltage and thesignal voltage.

In such a manner, according to the CDS, it is possible to significantlyreduce kTC noise generated during resetting each unit pixel cell. Thisis one reason why CMOS solid-state imaging devices have been mainlyused.

The following description will be given of problems of theabove-described related art which have been found by the presentinventor.

All signal charge generated in the PD portion, which detects an opticalsignal, is transferred to the FD. This transfer is herein referred to as“complete transfer”. In order to realize the complete transfer, thereare problems in that, for example, the manufacturing process iscomplicated and the manufacturing cost increases.

Also, a solid-state imaging device that is capable of performingnon-distortion image capture on an object moving at high speed, has beenproposed recently. For example, Japanese Unexamined Patent ApplicationPublication No. 2008-28516 (herein referred to as “Patent Document 2”)discloses a solid-state imaging device having unit pixel cells thatrealize a global shutter.

In the solid-state imaging device in Patent Document 2, charges in PDportions in all unit pixel cells are transferred to FDs at a time.Thereafter, a signal voltage is sequentially read for each row.Consequently, reset voltages cannot be read before signal voltages inthe FDs are read. Thus, when CDS is applied to the solid-state imagingdevice in Patent Document 2, the signal voltages in the FDs are read,then the FDs are reset, the reset voltages are read, and a differencebetween the signal voltage and the reset voltage is obtained. In thiscase, however, reset noise contained in the signal voltage and resetnoise contained in the reset voltage has no correlation, it is difficultto remove the reset noise. Consequently, even when CDS is applied to thesolid-state imaging device in Patent Document 2, random noise increases,compared with the above-described reading using the CDS.

Nowadays, the area per unit pixel cell decreases as a result of anincrease in the number of unit pixel cells, and the area of the PDportion tends to decrease. Consequently, the sensitivity may decline.Solid-state imaging devices that overcome this problem have also beenproposed. For example, Japanese Unexamined Patent ApplicationPublication No. 2014-78870 (Patent Document 3) discloses an organic CMOSsensor in which an organic photoelectric conversion layer is used in aPD portion. According to the organic CMOS sensor, the organicphotoelectric conversion layer, which serves as a light receivingportion, is provided above a reading circuit. Thus, even when the pixelsize decreases, the area of the light receiving portion can beincreased. This makes it possible to realize a high sensitivity.

In organic CMOS sensors, since the PD portion and a semiconductor layerare electrically connected through a metal wire, the signal chargecannot be completely transferred. Accordingly, a typical method that hasbeen employed is a method in which the PD portion and the FD areelectrically connected to read a voltage due to signal chargeaccumulated in the PD portion. This can suppress noise and afterimagegeneration due to the incomplete transfer.

Specifically, the voltage of the FD varies depending on the amount ofsignal charge accumulated in the FD during exposure. The voltage of theFD when the signal charge is accumulated is read as a signal voltage.Thereafter, the signal charge accumulated in the FD is reset, and thevoltage of the FD at this point in time is read as a reset voltage.Then, the difference between the signal voltage and the reset voltage isobtained.

In this case, however, there is no correlation between reset noisecontained in the reset voltage and reset noise contained in the signalvoltage. Thus, the reset noise cannot be eliminated. Consequently, theamount of random noise increases, compared with the above-describedreading using the above-described CDS.

In view of such problems, the present inventor has conceived an imagingdevice having a novel structure. An overview of one aspect of thepresent disclosure will be described as the following items.

[Item 1] An imaging device comprising a unit pixel cell comprising: aphotoelectric converter that generates an electric signal throughphotoelectric conversion of incident light; and a signal detectioncircuit that detects the electric signal, the signal detection circuitcomprising a first transistor that amplifies the electric signal, asecond transistor that selectively transmits output of the firsttransistor to outside of the unit pixel cell, and a feedback circuitthat forms a feedback loop through which the electric signal isnegatively fed back, the feedback loop not passing through the firsttransistor.

According to this configuration, the reset noise can be suppressedeffectively.

[Item 2] In the imaging device according to item 1, the first transistormay be provided out of the feedback loop.

[Item 3] An imaging device comprising a unit pixel cell comprising: aphotoelectric converter that generates an electric signal throughphotoelectric conversion of incident light; and a signal detectioncircuit that detects the electric signal, the signal detection circuitcomprising a first transistor that amplifies the electric signal, asecond transistor that selectively transmits output of the firsttransistor to outside of the unit pixel cell, and a feedback circuitthat forms a feedback loop through which the electric signal isnegatively fed back, the feedback loop passing through the firsttransistor, the feedback circuit comprising a third transistor providedon the feedback loop, a gate of the third transistor being connected toa voltage control circuit that outputs at least three voltages differentfrom each other.

According to this configuration, the reset noise can be suppressedeffectively.

[Item 4] In the imaging device according to item 3, the first transistormay be provided on the feedback loop.

[Item 5] The imaging device according to item 3 or 4 may furthercomprise a constant current source provided on an output side of thefirst transistor.

According to this configuration, it is possible to increase the band ofthe first transistor (an amplification transistor), and it is possibleto perform noise suppression at high speed.

[Item 6] In the imaging device according to one of items 1 to 5, thesignal detection circuit may further comprise a reset transistor thatinitializes the electric signal.

According to this configuration, it is possible to initialize theelectric signal in the photoelectric converter, and it is possible tosuppress the reset noise at high speed.

[Item 7] In the imaging device according to one of items 1 to 5, thesignal detection circuit may amplify the electric signal with a firstamplification factor in a period when the feedback circuit forms thefeedback loop, and the signal detection circuit may amplify the electricsignal with a second amplification factor different from the firstamplification factor in a period when the feedback circuit does not formthe feedback loop.

According to this configuration, it is possible to improve a noisesuppression ratio during noise suppression, and it is possible toperform a stable operation during reading.

[Item 8] In the imaging device according to item 6, the reset transistormay be provided on the feedback loop.

According to this configuration, the reset noise can be effectivelysuppressed with a small area and at high speed.

[Item 9] In the imaging device according to item 8, the reset transistormay function as a negative feedback transistor.

According to this configuration, the reset noise can be effectivelysuppressed with a small area and at high speed.

[Item 10] In the imaging device according to one of items 1 to 9, thefeedback circuit may comprise a first capacitor and a second capacitorconnected in series to the first capacitor, and the first capacitor andthe second capacitor may be provided between the photoelectric converterand a reference voltage.

According to this configuration, the reset noise can be suppressed moreeffectively.

[Item 11] In the imaging device according to item 10, the firstcapacitor may be provided on the feedback loop.

According to this configuration, the reset noise can be suppressed moreeffectively.

[Item 12] In the imaging device according to item 10, the firstcapacitor may have a first capacitance value, and the second capacitormay have a second capacitance value larger than the first capacitancevalue.

According to this configuration, the capacitance of the second capacitoris increased as long as the area permits, so that the reset noise can besuppressed more efficiently.

[Item 13] In the imaging device according to item 6, one end of thereset transistor may be connected to the photoelectric converter withoutforming a part of the feedback loop of the feedback circuit.

According to this configuration, a signal in the photoelectric convertercan be reset to an arbitrary level.

[Item 14] In the imaging device according to items 6 and 13, one end ofthe reset transistor may be connected to an output end of the firsttransistor.

According to this configuration, the signal in the photoelectricconverter can be reset by an output signal of the first transistorwithout provision of a reference voltage. Since changes in the signal inthe photoelectric converter before and after the reset transistor isturned off can be reduced, it is possible to perform noise suppressionat higher speed.

[Item 15] The imaging device according to items 6 and 13 may furtherhave a band control transistor that controls a band of the feedbackcircuit. The band control transistor may form a part of the feedbackloop of the feedback circuit, and one end of the reset transistor may beconnected to the band control transistor.

According to this configuration, the signal in the photoelectricconverter can be reset by an output signal of the band controltransistor without provision of a reference voltage. Since changes inthe signal in the photoelectric converter before and after the resettransistor is turned off can be reduced, it is possible to perform noisesuppression at higher speed.

[Item 16] The imaging device according to one of items 1 to 5 mayfurther have a signal reading line connected to the second transistor,and the second transistor may further have a first period in which anoutput end of the first transistor and the signal reading line areelectrically disconnected from each other and a second period in whichan output end of the first transistor and the signal reading line areelectrically connected to each other.

According to this configuration, since the output end of the firsttransistor and the signal reading line are not connected in the firstperiod, it is possible to perform high-speed noise cancelling withoutdepending on the time constant of the reading line.

[Item 17] The imaging device according to one of items 1 to 5 mayfurther have a band control transistor that controls a band of thefeedback circuit. The band control transistor may be controlled so as tocontinuously change from an arbitrary first band to an arbitrary secondband.

According to this configuration, stable noise cancelling can beperformed.

[Item 18] A camera system has an imaging device according to one ofitems 1 to 17, an optical system that forms a subject image on theimaging device, and a signal processor that processes a signal outputfrom the imaging device.

According to this camera system, the reset noise during reading can beappropriately suppressed, and a favorable image can be acquired.

Embodiments of the present disclosure will be described below withreference to the accompanying drawings. It is to be noted that thepresent disclosure is not limited to the embodiments described below.Changes are also possible within a scope that does not depart from ascope in which advantages of the present disclosure are obtained. Inaddition, one embodiment can be combined with another embodiment. In thefollowing description, the same or similar constituent elements aredenoted by the same reference numerals. A redundant description may alsobe omitted.

In first to fifth embodiments described below, it is assumed thattransistors, except for some transistors, in a signal reading circuitare, in principle, n-channel metal-oxide semiconductor (NMOS)transistors. Naturally, p-channel metal-oxide semiconductor (PMOS)transistors may be used in place of the NMOS transistors. In such acase, the polarity of each control signal is inverted. NMOS transistorsand PMOS transistors may also be used in combination.

First Embodiment

The structure, functions, and a drive method of an imaging device 100according to a first embodiment will be described with reference toFIGS. 3 to 8. The imaging device 100 according to the present embodimenthas reading circuits 50A, each including three transistors.

(Structure of Imaging Device 100)

First, the structure of the imaging device 100 will be described withreference to FIG. 3.

FIG. 3 schematically illustrates an exemplary circuit configuration ofthe imaging device 100 according to the present embodiment. The imagingdevice 100 is, for example, a stack-type imaging device and has aphotoelectric conversion layer stacked on a semiconductor substrate. Theimaging device 100 has a plurality of unit pixel cells 110A andperipheral circuitry.

The unit pixel cells 110A are two dimensionally arranged to form aphotosensitive area (pixel area). The unit pixel cells 110A may also bearranged one dimensionally. In this case, the imaging device 100 may bea line sensor.

In the illustrated example, the unit pixel cells 110A are arranged in arow direction and a column direction. The “row direction” and the“column direction” as used herein refer to directions in which a row anda column extend, respectively. That is, in the drawings, theperpendicular direction (the up-and-down direction) corresponds tocolumn direction, and the horizontal direction (the left-and-rightdirection) corresponds to the row direction.

The unit pixel cells 110A are connected to a power-supply wire 120. Apredetermined power-supply voltage is supplied to the unit pixel cells110A through the power-supply wire 120. The imaging device 100 has aphotoelectric conversion layer for photoelectrically converting incidentlight. The same constant voltage is supplied to the entire photoelectricconversion layer through an accumulation control line 130. However, whencontrol for suppressing variations or the like is performed, differentvoltages may be supplied to respective areas obtained by dividing thephotoelectric conversion layer. A detailed description of the unit pixelcells 110A is given later.

The peripheral circuitry includes a vertical scanning circuit (alsocalled a “row scanning circuit”) 141, column-signal processing circuits(also called “row-signal accumulation circuits”) 142, ahorizontal-signal reading circuit (also called a “column scanningcircuit”) 143, and constant current sources 144. One column-signalprocessing circuit 142 and one constant current source 144 may bearranged for each column of the two-dimensionally arranged unit pixelcells 110A.

One example of the configuration of the peripheral circuitry will bedescribed below.

The vertical scanning circuit 141 is connected to selection controlsignal lines (address signal lines) CON7 and band control signal linesCON1. The vertical scanning circuit 141 applies a predetermined voltageto each selection control signal line CON7 to thereby select, row byrow, the unit pixel cells 110A arranged in each row. Then, signalvoltages of the selected the unit pixel cells 110A are read, and pixelelectrodes described below are reset.

The unit pixel cells 110A arranged in each column are electricallyconnected to the column-signal processing circuit 142 through a verticalsignal line 170 corresponding to the column. The column-signalprocessing circuits 142 perform, for example, noise-suppression signalprocessing typified by correlated double sampling (CDS) andanalog-to-digital (A/D) conversion. The column-signal processingcircuits 142 are connected to the horizontal-signal reading circuit 143.The horizontal-signal reading circuit 143 reads signals from thecolumn-signal processing circuits 142 and outputs signals to ahorizontal common signal line 180.

Next, the structure of each unit pixel cell 110A will be described indetail with reference to FIGS. 4 to 6.

FIG. 4 schematically illustrates an exemplary circuit configuration ofeach unit pixel cell 110A in the imaging device 100 according to thepresent embodiment. The unit pixel cell 110A includes a photodetector 1,an amplifier 2, a band controller 3, a charge accumulator, and an outputselector 5. The photodetector 1 detects light to generate signal charge.Herein, the photodetector 1 may be referred to as a “photoelectricconverter”. The charge accumulator includes a node in which the signalcharge generated by the photodetector 1 is accumulated. This chargeaccumulator is hereinafter referred to as an “FD”.

The amplifier 2, the band controller 3, the FD, and the output selector5 constitute a reading circuit 50A. The photodetector 1 converts lightinto signal charge. The reading circuit 50A reads the signal chargegenerated by the photodetector 1.

Each of FIGS. 5A and 5B illustrates one example of the photodetector 1.The photodetector 1 may be constituted by, for example, an upperelectrode 1 a, a lower electrode 1 b, and an organic photoelectricconversion layer 1A interposed therebetween, as illustrated in FIG. 5A.When a reference voltage Vp is applied to the upper electrode 1 a, andone end of a node which forms the FD is connected to the lower electrode1 b, signal charge generated by the photodetector 1 can be accumulatedin the FD. As illustrated in FIG. 5B, the photodetector 1 may beimplemented by a photodiode 1B. When a ground voltage or the referencevoltage Vp is applied to one end of the photodiode 1B, and one end ofthe node which forms the FD is connected to another end of thephotodiode 1B, signal charge generated by the photodetector 1 can beaccumulated in the FD. The photodetector 1 may be another element havinga photoelectric conversion function.

A reference is made to FIG. 4 again. The FD is connected to thephotodetector 1 via a wiring layer. The FD accumulates the signal chargegenerated by the photodetector 1. The FD is further connected to aninput end of the amplifier 2. The amplifier 2 amplifies a signalcorresponding to the signal charge accumulated in the FD and outputs theamplified signal to the band controller 3 and the output selector 5.

The amplifier 2 and the band controller 3 form a loop (a feedbackcircuit) 30 via the FD. The band controller 3 includes a band controlcircuit 3A. At least three voltages that are different from one anotherare supplied from a voltage control circuit 99 to the band controlcircuit 3A. Since such voltages are supplied to the band control circuit3A, it has a band control function. The band control circuit 3A appliesband limitation to the signal output from the amplifier 2 and outputsthe resulting signal to the FD. In the feedback circuit 30, the signalread from the FD is amplified by the amplifier 2, the amplified signalis subjected to the band limitation by the band control circuit 3A, andthe resulting signal is fed back to the FD.

The output selector 5 is connected to a signal reading line 7. Thesignal reading line 7 is shared by at least two unit pixel cells. Thesignal amplified by the amplifier 2 is output to the signal reading line7 via the output selector 5. The signal reading line 7 corresponds tothe vertical signal line 170 illustrated in FIG. 3. A constant currentsource 6 corresponds to the constant current source 144 illustrated inFIG. 3.

The structure and functions of the reading circuit 50A will be describedin detail with reference to FIG. 6.

FIG. 6 schematically illustrates the circuit configuration of thereading circuit 50A. The feedback circuit 30 negatively feeds back asignal, which is input from the photodetector 1, to the FD via a firstamplification transistor 200.

The amplifier 2 has the first amplification transistor 200 and aswitching circuit 20, which includes a first switching element 11 and asecond switching element 12. It is assumed that transistors in thereading circuit 50A are NMOS transistors. The following description willbe given of a relationship of electrical connections in the readingcircuit 50A.

The FD is connected to the gate of the first amplification transistor200. The band controller 3 includes a first band control transistor 300.The output selector 5 includes a first selection transistor 500. One ofthe source and the drain of the first amplification transistor 200 isconnected to one of the source and drain of the first band controltransistor 300 and to one of the source and the drain of the firstselection transistor 500. Also, the other of the source and the drain ofthe first band control transistor 300 is connected to the FD. The firstband control transistor 300 and a capacitance component that isparasitic in the FD form a resistor-capacitor (RC) filter circuit.

The band control signal line CON1 is connected to the gate of the firstband control transistor 300. The state of the first band controltransistor 300 is determined by the voltage of the band control signalline CON1. For example, when the voltage of the band control signal lineCON1 is at a high level, the first band control transistor 300 is turnedon. As a result, the FD, the first amplification transistor 200, and thefirst band control transistor 300 form a feedback loop.

When the voltage of the band control signal line CON1 decreases,resistance components of the first band control transistor 300 increase.Consequently, the bandwidth of the first band control transistor 300decreases, and the frequency range of a signal that is fed backdecreases. When the voltage of the band control signal line CON1 reachesa low level, which is lower than the previous level, the first bandcontrol transistor 300 is turned off. As a result, no feedback loop isformed.

The other of the source and the drain of the first selection transistor500 is connected to the signal reading line 7. The gate of the firstselection transistor 500 is controlled through the selection controlsignal line CON7. The state of the first selection transistor 500 isdetermined by the voltage of the selection control signal line CON7. Forexample, when the voltage of the selection control signal line CON7 isat a high level, the first selection transistor 500 is turned on. As aresult, the first amplification transistor 200 and the signal readingline 7 are electrically connected to each other. When the voltage of theselection control signal line CON7 is at a low level, the firstselection transistor 500 is turned off. As a result, the firstamplification transistor 200 and the signal reading line 7 areelectrically isolated from each other.

The switching circuit 20 is connected to the other of the source and thedrain of the first amplification transistor 200. More specifically, theother of the source and the drain of the first amplification transistor200 is connected to a first voltage source VA1 via the first switchingelement 11. The other of the source and the drain of the firstamplification transistor 200 is also connected to a second voltagesource VA2 via the second switching element 12. The switching circuit 20is controlled with control signals V1 and V2 to thereby switch thevoltage, applied to the other of the source and the drain of the firstamplification transistor 200, between a voltage Va1 and a voltage Va2.The voltage Va1 of the first voltage source VA1 is, for example, aground voltage GND. The voltage Va2 of the second voltage source VA2 is,for example, a power-supply voltage VDD. The switching circuit 20 may beprovided for each unit pixel cell or may be shared by a plurality ofunit pixel cells in order to reduce the number of elements per unitpixel cell.

The constant current source 6 is connected to the signal reading line 7.When the first selection transistor 500 is on, the first selectiontransistor 500, the first amplification transistor 200, and the constantcurrent source 6 form a source follower circuit. A signal correspondingto the signal charge accumulated in the FD is output to the signalreading line 7 and is read to outside. The constant current source 6 maybe provided for each unit pixel sell or may be shared by a plurality ofunit pixel cells in order to reduce the number of elements per unitpixel cell.

(Operation of Imaging Device 100)

Next, an operation flow of the reading circuit 50A will be describedwith reference to FIG. 7.

FIG. 7 is a timing chart illustrating one example of the operation ofthe reading circuit 50A. The horizontal axis in each graph representstime, and the vertical axes represent a voltage of the band controlsignal line CON1, a voltage of the selection control signal line CON7,and a voltage VS of the other of the source and the drain of the firstamplification transistor 200 sequentially from the top in FIG. 7.

(Reset Period)

At time t1, the voltage of the selection control signal line CON7 is ata low level. Thus, the first selection transistor 500 is in an offstate, so that the first amplification transistor 200 and the signalreading line 7 are electrically isolated from each other. Also, at timet1, the voltage of the band control signal line CON1 is set to a highlevel to put the first band control transistor 300 into an on state.Also, at time t1, the first switching element 11 in the switchingcircuit 20 is in an on state, and the voltage Va1 (e.g., GND) is appliedto the other of the source and the drain of the first amplificationtransistor 200. As a result, the voltage of the FD becomes equal to areset voltage VRST.

In this case, the voltage of the band control signal line CON1 is set sothat the operating band of the first band control transistor 300 is afirst band, which is a wideband. This allows the voltage of the FD to beequal to the reset voltage VRST at high speed. The first band means theoperating band of the first band control transistor 300 whichcorresponds to a gate voltage at a high level. It should be noted that afrequency band of a signal passing between a source and a drain of atransistor is referred to as an operating band of the transistor.

In the present embodiment, this reset period is provided in order to setthe FD to have the reset voltage at high speed. However, if the drivetime allows extra time, an operation for setting the FD to have thereset voltage may be performed in a noise suppression period, describedbelow, without provision of the reset period.

(Noise Suppression Period)

Next, in the period of time t2 to time t4, the voltage of the bandcontrol signal line CON1 is set to a voltage between the high level andthe low level, for example, to an intermediate voltage. In this case,the operating band of the first band control transistor 300 is a secondband, which is narrower than the first band. The second band means theoperating band of the first band control transistor 300 when the gatevoltage is an intermediate voltage.

When the second band is made to be sufficiently smaller than theoperating band of the first amplification transistor 200, the noisesuppression effect increases. However, the period from time t2 to timet4 increases. Even when the second band is wider than the operating bandof the first amplification transistor 200, the noise suppression effectcan be obtained. Hence, a designer can arbitrarily design the secondband in accordance with an allowable time in the period of time t2 totime t4. The following description will be given assuming that thesecond band is sufficiently narrower than the operating band of thefirst amplification transistor 200.

When the second band is narrower than the operating band of the firstamplification transistor 200, the feedback circuit reduces thermal noisegenerated in the first band control transistor 300. When theamplification factor of the amplifier 2 is represented by −A, thethermal noise can be reduced to 1/(1+A)^(1/2).

The switching circuit 20 is set so that the other of the source and thedrain of the first amplification transistor 200 has the ground voltageGND. The designer can design the amplification factor of the amplifier 2so that it has an optimum value for the circuit system. Typically, A isset to a numerical value of 1 or greater and may be set to a numericalvalue of a few 10 s to several 100 s.

When the voltage of the band control line CON1 is set to a low level attime t4 to turn off the first band control transistor 300, kTC noisethat remains in the FD at this point in time is also reduced to1/(1+A)^(1/2), compared with a case in which no feedback is performed.

(Exposure/Reading Period)

At time t5, the voltage of the selection control signal line CON7 is setto the high level to turn on the first selection transistor 500. Also,the switching circuit 20 is controlled so that the voltage of the otherof the source and the drain of the first amplification transistor 200reaches the voltage Va2 (e.g., VDD). That is, the second switchingelement 12 is turned on, the voltage Va2 is applied to the other of thesource and the drain of the first amplification transistor 200. In thisstate, the first amplification transistor 200 and the constant currentsource 6 form a source follower circuit. The signal reading line 7 thenhas a voltage corresponding to the signal charge accumulated in the FD.At this point in time, the amplification factor of the source followercircuit is about 1.

At time t5, the voltage of the FD has changed relative to the resetvoltage VRST by an amount corresponding to a voltage according to signalcharge generated by the photodetector 1 in the period of time t4 to timet5. The amplifier 2 amplifies the voltage of the FD by an amplificationfactor of about 1, and the amplified signal is output to the signalreading line 7.

Random noise means fluctuations, that is, kTC noise, in the outputsignal when the signal charge generated by the photodetector 1 is 0. ThekTC noise is reduced to 1/(1+A)^(1/2) in the noise suppression period,and further, in an exposure/reading period, the resulting signal isoutput to the signal reading line 7 with an amplification factor ofabout 1. Thus, according to the present embodiment, it is possible toobtain favorable image data in which the random noise is reduced.

In addition, according to the present embodiment, CDS can also beperformed in order to cancel variations in the peripheral circuitry.Specifically, after the source follower circuit reads the signal voltageof the FD, the above-described reset operation is performed again. Afterthe reset operation is completed, the source follower circuit performsthe reading operation again before the photodetector 1 detects light.Thus, the reset voltage VRST can be read. It is possible to perform CDSby obtaining the difference between the signal voltage of the FD and thereset voltage.

In the present embodiment, since the source follower circuit reads thesignal in the FD in the exposure period, the amplification factor isabout 1. The present disclosure, however, is not limited to thisamplification factor, and the designer may vary the amplification factorin accordance with a signal-to-noise ratio (SNR) or a circuit rangeneeded for the system.

In the present embodiment, feedback for noise cancelling is performed ineach unit pixel cell of the unit pixel cells 110A. This allows the noisecancelling to be performed at high speed without being affected by thetime constant of the signal reading line 7.

Lastly, another control method for the band control signal line CON1will be described with reference to FIG. 8.

FIG. 8 is a timing chart illustrating another example of the operationof the reading circuit 50A. As illustrated in FIG. 8, the band controlsignal line CON1 may be controlled so that the first band controltransistor 300 changes gradually from the on state to the off stateacross a threshold voltage for the first band control transistor 300.Herein, such reset control is referred to as “taper reset”.

With the taper reset, even when the threshold voltages of the first bandcontrol transistors 300 in the unit pixel cells 110A in the imagingdevice 100 have variations, noise generated in all of the unit pixelcells 110A can be effectively suppressed. Also, a variation range of thevoltage applied to the band control signal line CON1 during the taperreset may be limited to a variation range of threshold voltages of thefirst band control transistors 300 in the unit pixel cells 110A. Thismakes it possible to reduce the time taken for the taper reset and makesit possible to perform the noise suppression at high speed.

Second Embodiment

The structure, functions, and a drive method of an imaging device 100according to a second embodiment will be described with reference toFIGS. 9 to 17. The imaging device 100 according to the presentembodiment differs from the first embodiment in that the imaging device100 has reading circuits 50B each including four transistors.

(Structure of Imaging Device 100)

The imaging Device 100 according to the present embodiment hastwo-dimensionally arranged unit pixel cells 110B and peripheralcircuitry, as in the first embodiment. The unit pixel cells 110B areconnected to the peripheral circuitry through various control lines.

FIG. 9 schematically illustrates an exemplary circuit configuration ofeach unit pixel cell 110B in the imaging device 100 according to thepresent embodiment. The unit pixel cell 110B includes a photodetector 1and a reading circuit 50B. The reading circuit 50B includes an amplifier2, a band controller 3′, an FD, and an output selector 5. The readingcircuit 50B reads signal charge generated by the photodetector 1.

The FD is connected to the photodetector 1 via a wiring layer. The FD isfurther connected to an input end of the amplifier 2. The amplifier 2amplifies a signal corresponding to signal charge accumulated in the FDand outputs the amplified signal to the band controller 3′ and theoutput selector 5.

The band controller 3′ includes a reset circuit 4A, which resets the FD,and a band control circuit 3B. At least three voltages that aredifferent from each other are supplied from a voltage control circuit 99to the band control circuit 3B. Since such voltages are supplied to theband control circuit 3B, it has a band control function. The bandcontrol circuit 3B applies band limitation to the signal output from theamplifier 2 and outputs the resulting signal to the FD. The signalcharge accumulated in the FD is reset by the reset circuit 4A. A signalread from the FD is amplified by the amplifier 2. The amplified signalis subjected to band limitation by the band control circuit 3B, and theresulting signal is fed back to the FD.

The output selector 5 is connected to the signal reading line 7. Thesignal reading line 7 is shared by at least two unit pixel cells. Thesignal amplified by the amplifier 2 is output to the signal reading line7 via the output selector 5.

FIG. 10 schematically illustrates one example of the circuitconfiguration of the reading circuit 50B. A feedback circuit 30′negatively feeds back a signal, which is input from the photodetector 1,to the FD via a first amplification transistor 200.

A reset circuit 4A in the band controller 3′ includes a reset transistor400. The band control circuit 3B includes a second band controltransistor 301, a first capacitor 9, and a second capacitor 10. The term“capacitor” as used herein refers to a structure in which a dielectric,such as an insulating film, is interposed between electrodes. The“electrodes” are not limited to electrodes made of metal and are widelyconstrued as including a polysilicon layer or the like. The electrodesmay be a part of a semiconductor substrate. The first capacitor 9 andthe second capacitor 10 may be, for example, metal-insulator-metal (MIM)capacitors or metal-insulator-semiconductor (MIS) capacitors.

An amplifier 2 has the first amplification transistor 200 and aswitching circuit 20 including a first switching element 11 and a secondswitching element 12. An output selector 5 includes a first selectiontransistor 500. The following description will be given of arelationship of electrical connections in the reading circuit 50B.

The gate of the first amplification transistor 200 is connected to theFD. One of the source and the drain of the first amplificationtransistor 200 is connected to one of the source and the drain of thesecond band control transistor 301 and to one of the source and thedrain of the first selection transistor 500. The other of the source andthe drain of the second band control transistor 301 is connected to oneend of the first capacitor 9. A reference voltage VR1 is applied to theother end of the first capacitor 9. Thus, the second band controltransistor 301 and the first capacitor 9 form an RC filter circuit.

The other of the source and the drain of the second band controltransistor 301 is also connected to one end of the second capacitor 10.The other end of the second capacitor 10 is connected to the FD. A nodeformed among the second band control transistor 301, the first capacitor9, and the second capacitor 10 is herein referred to as an “RD”.

A band control signal line CON3 is connected to the gate of the secondband control transistor 301. The state of the second band controltransistor 301 is determined by the voltage of the band control signalline CON3. For example, when the voltage of the band control signal lineCON3 is at a high level, the second band control transistor 301 isturned on. At this point in time, the FD, the first amplificationtransistor 200, the second band control transistor 301, and the secondcapacitor 10 form a feedback loop (the feedback circuit 30′).

When the voltage of the band control signal line CON3 decreases,resistance components of the second band control transistor 301increase. Thus, the band of the second band control transistor 301 isnarrowed, and the frequency band of a signal that is fed back isnarrowed.

When the feedback loop is formed, a signal output from the second bandcontrol transistor 301 is attenuated by an attenuation circuit formed bythe second capacitor 10 and parasitic capacitance of the FD, and theresulting signal is fed back to the FD. When the capacitance of thesecond capacitor 10 is represented by Cc, and the parasitic capacitanceof the FD is represented by Cfd, an attenuation rate B is given byCc/(Cc+Cfd).

When the voltage of the band control signal line CON3 decreases furtherto reach the low level, the second band control transistor 301 is turnedoff, and no feedback loop is formed.

The FD is further connected to one of the source and the drain of thereset transistor 400. A reference voltage VR2 is applied to the other ofthe source and the drain of the reset transistor 400. The gate of thereset transistor 400 is connected to a reset control signal line CON2,and the state of the reset transistor 400 is determined by the voltageof the reset control signal line CON2. For example, when the voltage ofthe reset control signal line CON2 is at a high level, the resettransistor 400 is turned on, and the FD is reset to have the referencevoltage VR2.

The other of the source and the drain of the first selection transistor500 is connected to the signal reading line 7. The gate of the firstselection transistor 500 is connected to the selection control signalline CON7, and the state of the first selection transistor 500 isdetermined by the voltage of the selection control signal line CON7. Forexample, when the voltage of the selection control signal line CON7 isat a high level, the first selection transistor 500 is turned on, andthe first amplification transistor 200 and the signal reading line 7 areelectrically connected to each other. When the voltage of the selectioncontrol signal line CON7 is at a low level, the first selectiontransistor 500 is turned off. As a result, the first amplificationtransistor 200 and the signal reading line 7 are electrically isolatedfrom each other.

The switching circuit 20 is connected to the other of the source and thedrain of the first amplification transistor 200. More specifically, theother of the source and the drain of the first amplification transistor200 is connected to a first voltage source VA1 via the first switchingelement 11. The other of the source and the drain of the firstamplification transistor 200 is also connected to a second voltagesource VA2 via the second switching element 12. The switching circuit 20is controlled with control signals V1 and V2 to thereby switch thevoltage, which is applied to the other of the source and the drain ofthe first amplification transistor 200, between a voltage Va1 and avoltage Va2. The voltage Va1 of the first voltage source VA1 is, forexample, a ground voltage GND. The voltage Va2 of the second voltagesource VA2 is, for example, a power-supply voltage VDD. The switchingcircuit 20 may be provided for each unit pixel cell or may be shared bya plurality of unit pixel cells in order to reduce the number ofelements per unit pixel cell.

The constant current source 6 is connected to the signal reading line 7.When the first selection transistor 500 is on, the first selectiontransistor 500, the first amplification transistor 200, and the constantcurrent source 6 form a source follower circuit. A signal correspondingto the signal charge accumulated in the FD is output to the signalreading line 7 and is output to outside of the unit pixel cell. Theconstant current source 6 may be provided for each unit pixel cell ormay be shared by a plurality of unit pixel cells in order to reduce thenumber of elements per unit pixel cell.

Next, an operation flow of the reading circuit 50B will be describedwith reference to FIG. 11.

FIG. 11 is a timing chart illustrating one example of the operation ofthe reading circuit 50B. The horizontal axis in each graph representstime, and the vertical axes represent a voltage of the reset controlsignal line CON2, a voltage of the band control signal line CON3, avoltage of the selection control signal line CON7, and a voltage VS ofthe other of the source and the drain of the first amplificationtransistor 200 sequentially from the top in FIG. 11.

(Reset Period)

At time t11, the voltage of the selection control signal line CON7 is atthe low level. Thus, the first selection transistor 500 is in an offstate, and the first amplification transistor 200 and the signal readingline 7 are electrically isolated from each other. Also, at time t11, thevoltage of the band control signal line CON3 is set to the high level toput the second band control transistor 301 into an on state. Also, attime t11, the first switching element 11 in the switching circuit 20 isin an on state, and the voltage Va1 (e.g., GND) is applied to the otherof the source and the drain of the first amplification transistor 200.In addition, at time t11, the voltage of the reset control signal lineCON2 is set to the high level to turn on the reset transistor 400, sothat the FD is reset, and the voltage of the FD reaches the referencevoltage VR2.

At time t12, the voltage of the reset control signal line CON2 is set tothe low level to turn off the reset transistor 400. At this point intime, the reading circuit 50B forms a feedback circuit in which theamplification factor is −A×B. Thus, kTC noise that is generated in theFD when the reset transistor 400 is turned off is reduced to 1/(1+A×B).It is possible to suppress kTC noise at high speed by setting thevoltage of the band control signal line CON3 to a high level so that theoperating band of the second band control transistor 301 is a firstband, which is a wideband.

(Noise Suppression Period)

In the period of time t13 to time t15, the voltage of the band controlsignal line CON3 is set to a voltage between the high level and the lowlevel, for example, to an intermediate voltage. In this case, theoperating band of the second band control transistor 301 is a secondband, which is narrower than the first band.

When the second band is made to be sufficiently narrower than theoperating band of the first amplification transistor 200, the noisesuppression effect increases, but the period from t13 to time t15increases. Even when the second band is wider than the operating band ofthe first amplification transistor 200, the noise suppression effect canbe obtained. The designer can arbitrarily design the second band inaccordance with an allowable time in the period of time t13 to time t15.The second band is hereinafter regarded as a band that is sufficientlynarrower than the operating band of the first amplification transistor200.

When the second band is narrower than the operating band of the firstamplification transistor 200, the feedback circuit 30′ reduces thermalnoise generated in the second band control transistor 301 to1/(1+A×B)^(1/2). In this state, when the voltage of the band controlsignal line CON3 is set to the low level at time t15 to turn off thesecond band control transistor 301, kTC noise that remains in the FD atthis point in time has the value of the square root of the sum of thesquare of kTC noise due to the reset transistor 400 and the square ofkTC noise due to the second band control transistor 301.

When the capacitance of the first capacitor 9 is Cs and there is nosuppression due to the feedback, kTC noise of the second band controltransistor 301 is (Cfd/Cs)^(1/2) times as large as the kTC noise of thereset transistor 400. In view of this point, when compared with a casein which no feedback is performed, kTC noise in a case where thefeedback is performed is reduced to {1+(1+A×B)×Cfd/Cs}^(1/2)/(1+A×B).

(Exposure/Reading Period)

The voltage of the selection control signal line CON7 is set to the highlevel at time t16 to turn on the first selection transistor 500, and theswitching circuit 20 is controlled so that the voltage of the other ofthe source and the drain of the first amplification transistor 200reaches the voltage Va2 (e.g., VDD). In this state, the firstamplification transistor 200 and the constant current source 6 form asource follower circuit. The signal reading line 7 has a voltagecorresponding to the signal charge accumulated in the FD. At this pointin time, the amplification factor of the source follower circuit isabout 1.

At time t16, the voltage of the FD has changed from the reset voltage(VR2) by an amount corresponding to signal charge generated by thephotodetector 1 in the period of time t15 to t16. The amplifier 2amplifies the voltage of the FD by an amplification factor of about 1and outputs the resulting voltage to the signal reading line 7.

Random noise means fluctuations, that is, kTC noise, in the outputsignal when the signal charge generated by the photodetector 1 is 0. ThekTC noise is reduced to {1+(1+A×B)×Cfd/Cs}^(1/2)/(1+A×B) in the noisesuppression period, and further, the resulting signal is output to thesignal reading line 7 in the exposure/reading period with anamplification factor of about 1. As a result, it is possible to obtainfavorable image data in which the random noise is reduced.

It is desirable that the capacitance Cs of the first capacitor 9 belarger than the capacitance Cc of the second capacitor 10. In thepresent embodiment, the random noise can be suppressed by increasing Csas long as the area permits. Typically, when the capacitance of thefirst capacitor 9 is increased, the random noise is reduced. However,since the signal level decreases when the FD converts the signal chargeinto a signal voltage, the SNR is not improved. In the presentembodiment, however, since the FD and the RD are separated by the secondcapacitor 10, the signal level is less likely to decrease even when thecapacitance of the first capacitor 9 is increased. As a result, only therandom noise is reduced, thus offering an advantage in that only the SNRis improved.

According to the present embodiment, CDS can also be performed in orderto cancel variations in the peripheral circuitry, as in the firstembodiment. Specifically, after the source follower circuit reads thesignal voltage of the FD, the above-described reset operation isperformed again. After the reset operation is completed, the sourcefollower circuit performs the reading operation again before thephotodetector 1 detects light. Thus, the reset voltage VRST can be read.It is possible to perform CDS by obtaining the difference between thesignal voltage of the FD and the reset voltage.

In the present embodiment, since the source follower circuit reads thesignal in the FD in the exposure period, the amplification factor isabout 1. The present disclosure, however, is not limited to thisamplification factor, and the designer may vary the amplification factorin accordance with an SNR or a circuit range needed for the system.

In the present embodiment, feedback for noise cancelling is performed ineach unit pixel cell of the unit pixel cells 110B. This allows the noisecancelling to be performed at high speed without being affected by thetime constant of the signal reading line 7. In addition, when thecapacitance of the capacitors arranged in each unit pixel cell isincreased, a greater noise suppression effect is obtained.

The following description will be given of modifications of theconfiguration and the operation of the reading circuit 50B according tothe present embodiment.

FIGS. 12 to 16 schematically illustrate other examples of the circuitconfiguration of the reading circuit 50B. Reading circuits 50B′illustrated in FIGS. 12 and 13 differ from the reading circuit 50Billustrated in FIG. 10 in that a voltage (an output voltage of theamplifier 2) of one of the source and the drain of the firstamplification transistor 200, instead of the reference voltage VR2, isapplied to the reset transistor 400. With such configurations, changesin the voltage of the FD before and after the reset transistor 400 isturned off can be reduced, and the noise suppression can be performed athigher speed.

In addition, a constant current source 8 may be provided in the unitpixel cell 110B, as illustrated in FIGS. 14 to 16. Such a configurationmakes it possible to widen the operating band of the first amplificationtransistor 200, thus making it possible to widen the band of the secondband control transistor 301. Accordingly, in a state in which the bandof the second band control transistor 301 is wider, the random noise canbe suppressed at higher speed.

Lastly, another control method for the band control signal line CON3will be described with reference to FIG. 17.

FIG. 17 is a timing chart illustrating another example of the operationof the reading circuit 50B. As illustrated in FIG. 17, the taper resetmay be applied, as in the first embodiment. That is, the band controlsignal line CON3 may be controlled so that the second band controltransistor 301 changes gradually from the on state to the off stateacross the threshold voltage therefor.

With such control, even when there are variations in the thresholdvoltages of the second band control transistors 301 in the unit pixelcells 110B included in the imaging device 100, it is possible toeffectively suppress noise that is generated in all the unit pixelcells. Also, a variation range of the voltage applied to the bandcontrol signal line CON3 during the taper reset may be limited to avariation range of the threshold voltages of the second band controltransistors 301 in the unit pixel cells. This makes it possible toreduce the time taken for the taper reset and makes it possible toperform the noise suppression at high speed.

Third Embodiment

The structure, functions, and a drive method of an imaging device 100according to a third embodiment will be described with reference toFIGS. 18 to 22B. The imaging device 100 according to the presentembodiment differs from the imaging device 100 according to the secondembodiment in that an output selector 5C in a reading circuit 50Cincludes a PMOS transistor serving as a selection transistor and isconnected to a switching circuit 40. The following description will bemainly given of differences from the second embodiment.

FIG. 18 schematically illustrates an exemplary circuit configuration ofeach unit pixel cell 110C in the imaging device 100 according to thepresent embodiment. Each unit pixel cell 110C includes a photodetector 1and the reading circuit 50C. The reading circuit 50C includes anamplifier 2, a band controller 3, an FD, and the output selector 5C. Theoutput selector 5C is connected to the switching circuit 40 through thesignal reading line 7.

The output selector 5C is connected to the signal reading line 7, whichis shared by at least two unit pixel cells. The output selector 5C has afunction for outputting a signal amplified by the amplifier 2 to thesignal reading line 7 and a function for supplying current to theamplifier 2. It is possible to switch between these functions.

FIG. 19A schematically illustrates the circuit configuration of thereading circuit 50C. A feedback circuit 30 negatively feeds back asignal, which is input from the photodetector 1, to the FD via a firstamplification transistor 200. One of the source and the drain of a thirdselection transistor 502 is connected to one of the source and the drainof the first amplification transistor 200. The other of the source andthe drain of the third selection transistor 502 is connected to thesignal reading line 7. In the present embodiment, the third selectiontransistor 502 has a polarity that is opposite to the polarity of thefirst amplification transistor 200. The first amplification transistor200 is an NMOS transistor, and the third selection transistor 502 is aPMOS transistor.

The gate of the third selection transistor 502 is connected to aselection control signal line CON8. The state of the third selectiontransistor 502 is determined by the voltage of the selection controlsignal line CON8. For example, when the voltage of the selection controlsignal line CON8 is at a low level, the third selection transistor 502is turned on, and the first amplification transistor 200 and the signalreading line 7 are electrically connected to each other. When thevoltage of the selection control signal line CON8 is at a high level,the third selection transistor 502 is turned off, and the firstamplification transistor 200 and the signal reading line 7 areelectrically isolated from each other.

When the voltage of the selection control signal line CON8 is a voltagebetween the low level and the high level, for example, an intermediatevoltage, the third selection transistor 502 operates as a current sourceand supplies current to the first amplification transistor 200. Theamount of the current is determined by the voltage of the selectioncontrol signal line CON8. A designer can design the reading circuit 50Cso that the amount of current reaches a desired amount.

The switching circuit 40 is connected to the signal reading line 7. Theswitching circuit 40 includes a third switching element 13, a fourthswitching element 14, voltage sources VB1 and VB2, and a constantcurrent source 6. One end of the constant current source 6 is connectedto the signal reading line 7 via the third switching element 13. Thevoltage source VB2 is connected to the signal reading line 7 via thefourth switching element 14. The voltage source VB1 is connected to theother end of the constant current source 6.

With control signals V3 and V4, it is possible to switch between thevoltage source VB2 and the constant current source 6 (the voltage sourceVB1) to be connected to the signal reading line 7. For example, avoltage Vb1 of the voltage source VB1 is a ground voltage GND, and avoltage Vb2 of the voltage source VB2 is a power-supply voltage VDD.

When the voltage source VB2 is connected to the signal reading line 7,and the voltage of the selection control signal line CON8 is a voltagebetween the low level and the high level, for example, is anintermediate voltage, the third selection transistor 502 operates as acurrent source. In this case, the third selection transistor 502 and thefirst amplification transistor 200 form an inverting amplifier circuit.

When the constant current source 6 is connected to the signal readingline 7, and the voltage of the selection control signal line CON8 is ata low level, the first amplification transistor 200 and the constantcurrent source 6 form a source follower circuit. In this case, thesignal in the FD is output to the signal reading line 7.

Although, in the present embodiment, the transistors included in thereading circuit 50C, except for the third selection transistor 502, areNMOS transistors, the polarities of the transistors may be inverted.That is, the third selection transistor 502 may be an NMOS transistor,and the other transistors may be PMOS transistors. All of thetransistors in the reading circuit 50C may be NMOS transistors or may bePMOS transistors.

A reference to FIG. 19B is made. FIG. 19B illustrates a modification ofthe configuration described above and illustrated in FIG. 19A. In thismodification, the switching circuit 40 has constant current sources 6Aand 6B. The output selector 5C also has a fourth selection transistor503. The polarity of the fourth selection transistor 503 is the same asthe polarity of the first amplification transistor 200 and so on. Thatis, the fourth selection transistor 503 is an NMOS transistor.

The gate of the fourth selection transistor 503 is connected to aselection control signal line CON9. The state of the fourth selectiontransistor 503 is determined by the voltage of the selection controlsignal line CON9. For example, when the voltage of the selection controlsignal line CON9 is at a high level, the fourth selection transistor 503is turned on, and the first amplification transistor 200 and the signalreading line 7 are electrically connected to each other. When thevoltage of the selection control signal line CON9 is at a low level, thefourth selection transistor 503 is turned off, and the firstamplification transistor 200 and the signal reading line 7 areelectrically isolated from each other.

In the configuration illustrated in FIG. 19A, the voltage of theselection control signal line CON8 is set to a voltage between the lowlevel and the high level, for example, to an intermediate voltage, tothereby cause the third selection transistor 502 to operate as a currentsource. In contrast, in this modification, when the fourth switchingelement 14 and the fourth selection transistor 503 are turned on,current is supplied from the constant current source 6B to the firstamplification transistor 200.

Next, an operation flow of the reading circuit 50C in FIG. 19A will bedescribed with reference to FIG. 20.

FIG. 20 is a timing chart illustrating one example of the operation ofthe reading circuit 50C. The horizontal axis in each graph representstime, and the vertical axes represent, sequentially from the top, avoltage of the reset control signal line CON2, a voltage of the bandcontrol signal line CON3, a voltage of the selection control signal lineCON8, and a voltage VS of the other of the source and the drain of thefirst amplification transistor 200 which is connected to the switchingcircuit 20.

(Reset Period)

At time t21, the voltage of the selection control signal line CON8 isset to a voltage between the low level and the high level, for example,to an intermediate voltage. Also, the switching circuit 40 is controlledso that the voltage source VB2 is connected to the signal reading line7. Also, the voltage of the band control signal line CON3 is set to thehigh level to turn on the second band control transistor 301. At timet21, the other of the source and the drain of the first amplificationtransistor 200 is connected to the first voltage source VA1. The voltageVa1 of the first voltage source VA1 is, for example, a ground voltageGND. In addition, at time t21, the voltage of the reset control signalline CON2 is set to the high level to turn on the reset transistor 400,so that the FD is reset. As a result, the voltage of the FD becomesequal to the reference voltage VR2.

At time t22, the voltage of the reset control signal line CON2 is set tothe low level to turn off the reset transistor 400. At this point intime, the reading circuit 50C forms a feedback loop with anamplification factor of −A×B. Thus, the kTC noise in the FD when thereset transistor 400 is turned off is reduced to 1/(1+A×B). The voltageof the band control signal line CON3 is set so that the operating bandof the second band control transistor 301 is a first band, which is awideband. This makes it possible to suppress noise at high speed.

(Noise Suppression Period)

In the period of time t23 to time t25, the voltage of the band controlsignal line CON3 is set to a voltage between the high level and the lowlevel, for example, to an intermediate voltage. In this case, theoperating band of the second band control transistor 301 is a secondband, which is narrower than the first band.

When the second band is made to be sufficiently narrower than theoperating band of the first amplification transistor 200, the noisesuppression effect increases, but the period from time t23 to time t25increases. Even when the second band is higher than the operating bandof the first amplification transistor 200, the noise suppression effectis obtained. Hence, the designer can arbitrarily design the second bandin accordance with an allowable time in the period of time t23 to timet25. The second band is hereinafter regarded as a band that issufficiently narrower than the operating band of the first amplificationtransistor 200.

When the second band is narrower than the operating band of the firstamplification transistor 200, the feedback circuit 30 reduces thermalnoise generated in the second band control transistor 301 to1/(1+A×B)^(1/2). In this state, when the voltage of the band controlsignal line CON3 is set to the low level at time t25 to turn off thesecond band control transistor 301, kTC noise that remains in the FD atthis point in time has the value of the square root of the sum of thesquare of kTC noise due to the reset transistor 400 and the square ofkTC noise due to the second band control transistor 301.

When the capacitance of the first capacitor 9 is Cs and there is nosuppression due to the feedback, kTC noise of the second band controltransistor 301 is (Cfd/Cs)^(1/2) times as large as the kTC noise of thereset transistor 400. In view of this point, when compared with a casein which no feedback is performed, kTC noise in a case where thefeedback is performed is reduced to {1+(1+A×B)×Cfd/Cs}^(1/2)/(1+A×B). Attime t25, the voltage of the selection control signal line CON8 is setto the high level to turn off the third selection transistor 502. As aresult, the first amplification transistor 200 and the signal readingline 7 are electrically isolated from each other.

As in the operation flow described in the second embodiment andillustrated in FIG. 17, the taper reset may also be applied. That is, inthe period of time t23 to time t24, the band control signal line CON3may also be controlled so that the second band control transistor 301changes gradually from the on state to the off state across thethreshold voltage therefor.

With such control, even when there are variations in the thresholdvoltages for the second band control transistors 301 in the unit pixelcells 110C included in the imaging device 100, it is possible toeffectively suppress noise that is generated in all the unit pixelcells. Also, a variation range of the voltage applied to the bandcontrol signal line CON3 during the taper reset may be limited to avariation range of the threshold voltages of the second band controltransistors 301 in the unit pixel cells. This makes it possible toreduce the time taken for the taper reset and makes it possible toperform the noise suppression at high speed.

(Exposure/Reading Period)

At time t26, the voltage of the selection control signal line CON8 isset to the low level to turn on the third selection transistor 502, andthe switching circuit 20 is controlled so that the voltage of the otherof the source and the drain of the first amplification transistor 200reaches the voltage Va2 (e.g., VDD). The switching circuit 40 is alsocontrolled so that the constant current source 6 is connected to thesignal reading line 7. In this state, the first amplification transistor200 and the constant current source 6 form a source follower circuit.The signal reading line 7 has a voltage corresponding to the signalcharge accumulated in the FD. At this point in time, the amplificationfactor of the source follower circuit is about 1.

At time t26, the voltage of the FD has changed relative to the resetvoltage (VR2) by an amount corresponding to a voltage according to thesignal charge generated by the photodetector 1 in the period of time t25to time t26. The amplifier 2 amplifies the voltage of the FD by anamplification factor of about 1 and outputs the resulting voltage to thesignal reading line 7.

The kTC noise is reduced to {1+(1+A×B)×Cfd/Cs}^(1/2)/(1+A×B) in thenoise suppression period, and further, the resulting signal is output tothe signal reading line 7 in the exposure/reading period with anamplification factor of about 1. Thus, it is possible to obtain goodimage data in which the random noise is reduced.

In the present embodiment, the random noise can be suppressed byincreasing Cs as long as the area permits, as in the second embodiment.Typically, when the capacitance of the first capacitor 9 is increased,the random noise decreases. However, since the signal level decreaseswhen the FD converts the signal charge into a signal voltage, the SNR isnot improved. In the present embodiment, however, since the FD and theRD are separated by the second capacitor 10, the signal level is lesslikely to decrease even when the capacitance of the first capacitor 9 isincreased. As a result, only the random noise is reduced, thus offeringan advantage in that only the SNR is improved.

According to the present embodiment, CDS can also be performed in orderto cancel variations in the peripheral circuitry, as in the secondembodiment. Specifically, after the source follower circuit reads thesignal voltage of the FD, the above-described reset operation isperformed again. After the reset operation is completed, the sourcefollower circuit performs the reading operation again before thephotodetector 1 detects light. Thus, the reset voltage VRST can be read.It is possible to perform CDS by obtaining the difference between thesignal voltage of the FD and the reset voltage.

Also, feedback for noise cancelling is performed in each of the unitpixel cells of the unit pixel cells 110C. This allows the noisecancelling to be performed at high speed without being affected by thetime constant of the signal reading line 7. In addition, when thecapacitance of the capacitors arranged in each unit pixel cell 110C isincreased, a greater noise suppression effect is obtained.

Also, in the present embodiment, since the source follower circuit readsthe signal in the FD in the exposure period, the amplification factor isabout 1. The present disclosure, however, is not limited to thisamplification factor, and the designer may vary the amplification factorin accordance with an SNR or a circuit range needed for the system.

The following description will be given of modifications of theconfiguration and the operation of the reading circuit 50C according tothe present embodiment.

FIGS. 21A and 22A schematically illustrate other circuit configurationsof the reading circuit 50C. In reading circuits 50C′ illustrated inFIGS. 21A and 22A, a voltage (an output voltage of the amplifier 2) ofone of the source and the drain of the first amplification transistor200, instead of the reference voltage VR2, is applied to the resettransistor 400. In this point, the reading circuits 50C′ illustrated inFIGS. 21A and 22A differ from the reading circuit 50C illustrated inFIG. 19A. According to the configurations illustrated in FIGS. 21A and22A, since changes in the voltages of the FD before and after the resettransistor 400 is turned off can be reduced, the noise suppression canbe performed at higher speed.

The configuration in which the switching circuit 40 includes theconstant current source 6B, the configuration being described above withreference to FIG. 19B, can also be applied to the configurationsillustrated in FIGS. 21A and 22A. FIG. 21B illustrates a modification ofthe configuration illustrated in FIG. 21A, and FIG. 22B illustrates amodification of the configuration illustrated in FIG. 22A. In each ofthe modifications, the switching circuit 40 has a constant currentsource 6B in addition to a constant current source 6A. The outputselector 5C also has a fourth selection transistor 503, which is an NMOStransistor. In the configurations illustrated in FIGS. 21A and 22A,current can be supplied from the constant current source 6B to the firstamplification transistor 200 by turning on the fourth switching element14 and the fourth selection transistor 503, as in the configurationillustrated in FIG. 19B.

Fourth Embodiment

The structure, functions, and a drive method of an imaging device 100according to a fourth embodiment will be described with reference toFIGS. 23 to 27. The imaging device 100 according to the presentembodiment has the following differences from the imaging devices 100according to the first to third embodiments. First, an amplifier 2A in areading circuit 50D has an amplification function and a band controlfunction. Second, the amplifier 2A returns its output to its input endto thereby apply negative feedback by using its own amplificationfunction (with an amplification factor of −A) while performing bandcontrol, to reduce the reset noise to 1/(1+A)^(1/2).

FIG. 23 schematically illustrates an exemplary circuit configuration ofeach unit pixel cell 110D in the imaging device 100 according to thepresent embodiment. Each unit pixel cell 110D includes a photodetector 1and a reading circuit 50D. The reading circuit 50D includes an amplifier2A, an FD, and an output selector 5B. The output selector 5B isconnected to a constant current source 6 through a signal reading line 7and is driven by current from the constant current source 6. Theamplifier 2A amplifies a signal corresponding to signal chargeaccumulated in the FD and performs band control in order to suppress kTCnoise generated in the FD.

The structure and the functions of the reading circuit 50D will bedescribed in detail with reference to FIG. 24.

FIG. 24 schematically illustrates one example of the circuitconfiguration of the reading circuit 50D. The amplifier 2A includes asecond amplification transistor 201, and the output selector 5B includesa fourth amplification transistor 203 and a second selection transistor501. The following description will be given of a relationship inelectrical connections in the reading circuit 50D.

The gate of the second amplification transistor 201 and one of thesource and the drain thereof are connected to the FD. The other of thesource and the drain of the second amplification transistor 201 isconnected to a fourth control signal line CON4. The second amplificationtransistor 201 amplifies a signal voltage corresponding to signal chargeaccumulated in the FD.

The FD is connected to the gate of the fourth amplification transistor203. One of the source and the drain of the fourth amplificationtransistor 203 is connected to a power-supply voltage VDD or a referencevoltage. The other of the source and the drain of the fourthamplification transistor 203 is connected to one of the source and thedrain of the second selection transistor 501. The gate of the secondselection transistor 501 is connected to the seventh control signal lineCON7 which selects a row to be read out. The other of the source and thedrain of the second selection transistor 501 is connected to theconstant current source 6 through the signal reading line 7. In such amanner, the fourth amplification transistor 203, the second selectiontransistor 501, and the constant current source 6 form a source followercircuit. The second selection transistor 501 selectively outputs anoutput signal of the second amplification transistor 201 to outsidethrough the signal reading line 7.

The gate of the second amplification transistor 201 and one of thesource and the drain of the second amplification transistor 201respectively correspond to an input end and an output end of theamplifier 2A. Thus, connection of the output end of the amplifier 2A tothe input end thereof forms a feedback loop. Thus, the feedback circuit30 negatively feeds back a signal, which is input from the photodetector1, to the FD without involvement of the fourth amplification transistor203.

Next, an operation flow of the reading circuit 50D will be describedwith reference to FIG. 25.

FIG. 25 is a timing chart illustrating one example of the operation ofthe reading circuit 50D. The horizontal axis in each graph representstime, and the vertical axes represent a voltage of the fourth controlsignal line CON4 and a voltage of the seventh control signal line CON7sequentially from the top in FIG. 25.

(Reset Period)

At time t28, the voltage of the seventh control signal line CON7 is atthe low level, so that the second selection transistor 501 is turnedoff. That is, the signal reading line 7 and the fourth amplificationtransistor 203 are electrically isolated from each other. In this state,the voltage of the fourth control signal line CON4 is set to a firstreference voltage so that the FD has a voltage that is equal or close toa desired reset voltage VRST. At this point in time, the band of thesecond amplification transistor 201 is set to a third band, which is awideband. As a result, the FD, the gate of the second amplificationtransistor 201, and one of the source and the drain of the secondamplification transistor 201 are set to have a desired voltage at highspeed. The third band means a band corresponding to the first referencevoltage.

As the voltage of the FD gets closer to the reset voltage VRST, theamount of time taken for the noise suppression becomes smaller, thusmaking it possible to reduce the drive time. Accordingly, it isdesirable that the voltage of the fourth control signal line CON4 be setso that the voltage of the FD is equal or close to the reset voltageVRST. However, the setting value of the voltage of the fourth controlsignal line CON4 is not limited to that value, if the drive time allowsextra time. In the reset period, the second amplification transistor 201functions as a reset transistor.

(Noise Suppression Period)

In the period of time t29 to time t31, the seventh control signal lineCON7 remains at the low level, and the second selection transistor 501is in the off state. That is, the signal reading line 7 and the fourthamplification transistor 203 remain, electrically isolated from eachother. In this state, the voltage of the fourth control signal line CON4is set to a second reference voltage. As a result, the state of thesecond amplification transistor 201 changes gradually from the on stateto the off state. During the change, kTC noise is generated in thesecond amplification transistor 201. The kTC noise depends on acapacitance Cfd that is parasitic in the FD to which one of the sourceand the drain of the second amplification transistor 201 is connected.Thus, the noise is suppressed using the feedback loop involving thesecond amplification transistor 201. In the noise suppression period,the second amplification transistor 201 functions as a negative feedbacktransistor.

When the second reference voltage is set to a voltage with which thesecond amplification transistor 201 changes quickly from the on state tothe off state, the band of reset noise that is generated increases toseveral terahertz. Thus, with the feedback loop involving the amplifier2A, it is difficult to suppress high-frequency noise that exceeds theband of the amplifier 2A. In the period of time t29 to time t31, thesecond reference voltage is set so that the band of the secondamplification transistor 201 is a fourth band, which is narrower thanthe third band. The fourth band means a band corresponding to the secondreference voltage. This allows the band of the second amplificationtransistor 201 to be limited to the band of the amplifier 2A formed bythe feedback loop involving the second amplification transistor 201. Inaddition, the reset noise generated in the second amplificationtransistor 201 can be efficiency suppressed in all bands.

After the noise is sufficiently reduced, at time t31, the voltage of thefourth control signal line CON4 is changed to a fourth reference voltageat which the second amplification transistor 201 is completely turnedoff. As a result, the feedback loop involving the second amplificationtransistor 201 is disconnected, and the voltage of the FD is stabilizedin a state in which noise is suppressed.

The taper reset described above with reference to FIGS. 8 and 17 mayalso be applied in the noise suppression period in the presentembodiment. FIG. 26 is a timing chart illustrating one example of theoperation of the reading circuit 50D when the taper reset is applied. Asillustrated in FIG. 26, in the period of time t29 to time t30, thevoltage of the fourth control signal line CON4 may be gradually changedso as to cross the threshold voltage in the range of the secondreference voltage to a third reference voltage. As a result of thechange, the second amplification transistor 201 changes gradually fromthe on state to the off state. In other words, in the period of time t29to time t30, the voltage of the fourth control signal line CON4 ischanged so that the band of the second amplification transistor 201changes gradually from the fourth band to a fifth band. The fifth bandmeans a band corresponding to the third reference voltage. The state ofthe second amplification transistor 201 is gradually changed from the onstate to the off state while the band of the second amplificationtransistor 201 is limited to the band of the amplifier 2A formed by thefeedback loop involving the second amplification transistor 201. As aresult, noise generated in the FD can be suppressed in all bands. Inthis case, the fourth band and the fifth band are narrower than thethird band. The second reference voltage and the third reference voltagemay include predetermined margins, considering variations in manufactureamong unit pixel cells.

(Exposure/Reading Period)

In a desired period when noise in the FD is sufficiently suppressed andthe voltage is stable, signal charge is accumulated in the FD.Thereafter, at time t32, the second selection transistor 501 is turnedon, so that the fourth amplification transistor 203 is electricallyconnected to the signal reading line 7. Thus, the fourth amplificationtransistor 203 and the constant current source 6 form a source followercircuit. The signal charge accumulated in the FD is amplified by thesource follower circuit, and the amplified signal is output to theperipheral circuitry (such as a CDS circuit and an A/D circuit) throughthe signal reading line 7.

When a noise suppression ratio and stability during reading areconsidered, it is desirable to maximize the amplification factor of theamplifier 2A. For example, it is desirable to set the amplificationfactor of the amplifier 2A to a value larger than the amplificationfactor of the amplifier (source follower) in the output selector 5B.

According to the present embodiment, CDS can also be performed in orderto cancel variations in the peripheral circuitry, as in the otherembodiments. More specifically, at time t32, after the source followercircuit reads the signal voltage of the FD, the above-described resetoperation is performed again. After the reset operation is completed,the source follower circuit performs the reset-voltage reading operationagain, before the photodetector 1 detects light. Thus, the reset voltageVRST can be read. It is possible to perform CDS by obtaining thedifference between the signal voltage of the FD and the reset voltage.

In the present embodiment, since the source follower circuit reads thesignal in the FD in the exposure period, the amplification factor isabout 1. The present disclosure, however, is not limited to thisamplification factor, and the designer may vary the amplification factorin accordance with an SNR or a circuit range needed for the system.

According to the present embodiment, the feedback for noise cancellingis completed in each unit pixel cell, as in the first to thirdembodiments. This allows the noise cancelling to be performed at highspeed without being affected by the time constant of the signal readingline 7. In addition, the amplifier 2A has both the amplificationfunction and the band control function. This makes it possible to reducethe area of each unit pixel cell and makes it possible to meet a demandfor a narrower unit pixel cell. This is a noteworthy feature of thepresent embodiment. Even in an imaging device having a small pixel area,it is possible to effectively suppress noise in the FD withoutincreasing the number of constituent elements.

In the present embodiment, in the reset period and the noise suppressionperiod, the second selection transistor 501 is turned off, so that thefourth amplification transistor 203 is isolated from the signal readingline 7. However, the present disclosure is not limited to thisoperation, and for example, the signal may be read at a timing differentfrom the above-described timing. In such a case, the reading may beperformed while the second selection transistor 501 is in the on state.If the drive time allows extra time, only the operations in the noisesuppression period and the exposure/reading period may be performed byomitting the reset period and without performing driving for reducing aconvergence time for suppressing the reset noise. The signal readingline 7 and/or the constant current source 6 may be provided for eachunit pixel cell or may be shared by a plurality of unit pixel cells.

The following description will be given of a modification of theconfiguration and the operation of the reading circuit 50D according tothe present embodiment. FIG. 27 schematically illustrates anotherexample of the circuit configuration of the reading circuit 50D. What isnoteworthy in the configuration of this modification is that theamplifier 2A includes a third capacitor 19 and a fourth capacitor 20, inaddition to a third amplification transistor 202. The gate of the thirdamplification transistor 202 is connected to the FD. One of the sourceand the drain of the third amplification transistor 202 is connected toa sixth control signal line CONE. The other of the source and the drainof the third amplification transistor 202 is connected to one end of thethird capacitor 19 and one end of the fourth capacitor 20. The other endof the third capacitor 19 is connected to the third reference voltageVR3. The other end of the fourth capacitor 20 is connected to the FD. Anode RD is formed among the third amplification transistor 202, thethird capacitor 19, and the fourth capacitor 20.

According to the configuration in this modification, the gate of thethird amplification transistor 202 and the other end of the fourthcapacitor 20 respectively correspond to an input end and an output endof the amplifier 2A. Connection of the output end to the input end ofthe amplifier 2A forms a negative feedback loop. When the amplificationfactor of the amplifier 2A is represented by −A, reset noise generatedby the third amplification transistor 202 can be reduced to1/(1+A)^(1/2).

A first advantage of this modification is that kTC noise generated bythe third amplification transistor 202 can be reduced to(kT/C3)^(1/2)<(kT/Cfd)^(1/2) by setting a capacitance C3 of the thirdcapacitor 19 to a value larger than the capacitance Cfd of the FD. Asecond advantage is that, when a capacitance C4 of the fourth capacitor20 is set to be smaller than the capacitance of the FD, the amount ofnoise in the FD can be attenuated to C4/(Cfd+C4) by divided voltages ofthe capacitance Cfd of the FD and the capacitance C4 of the fourthcapacitor 20.

An advantage obtained by this modification is specifically compared withan advantage of the configuration illustrated in FIG. 24. In theconfiguration illustrated in FIG. 24, when the gain of the amplifier 2Ais represented by A, and the gain of the second amplification transistor201 is represented by A′, the reset noise in the second amplificationtransistor 201 is reduced to 1/(1+A)^(1/2)=1/(1+A′)^(1/2). On the otherhand, in this modification, when the gain of the amplifier 2A isrepresented by A, and the gain of the third amplification transistor 202is represented by A′, the reset noise in the third amplificationtransistor 202 is reduced to1/(1+A)^(1/2)=1/[1+A′×{C4/(Cfd+C4)}×(C3/Cfd)]^(1/2). Thus, compared withthe configuration illustrated in FIG. 24, it is possible tosignificantly reduce the reset noise.

With respect to suppression of noise, typically, when the capacitance C3of the third capacitor 19 is increased, the random noise is reduced.However, when the FD converts signal charge to a signal voltage, thesignal level decreases, and consequently, the SNR is not improved.However, according to this modification, since the FD and the RD areseparated by the fourth capacitor 20, the signal level does not decreaseeven when the capacitance is increased. Hence, since only the randomnoise is suppressed, the SNR is improved.

Next, a reading operation in the imaging device 100 according to thismodification will be described while paying attention to differencesfrom the drive method illustrated in FIG. 25 or 26.

The sixth control signal line CON6 is connected to the amplifier 2A. Inprinciple, a signal that is the same as the signal in the fourth controlsignal line CON4 illustrated in FIG. 25 is input to the sixth controlsignal line CON6. A fifth reference voltage, instead of the sixthcontrol signal line CON6, may be set as the third reference voltage VR3,and the voltage of the other of the source and the drain of the thirdamplification transistor 202 may be changed. Alternatively, the node RDmay be directly controlled. In this case, the fifth reference voltagecorresponds to the second reference voltage.

Also, a voltage that changes gradually from the on state to the offstate across the threshold for the third amplification transistor 202may be input to the sixth control signal line CON6, as in the fourthcontrol signal line CON4 in FIG. 26. That is, in the period of time t29to time t30, the voltage of the sixth control signal line CON6 may begradually changed so as to cross the threshold voltage in the range ofthe second reference voltage to the third reference voltage.Alternatively, in the period of time t29 to time t30, instead of thesixth control signal line CON6, a voltage that changes from the fifthreference voltage to a sixth reference voltage may be set as the thirdreference voltage VR3, and the voltage of the other of the source andthe drain of the third amplification transistor 202 may be changed.Alternatively, the node RD may be directly controlled. In this case, thesixth reference voltage corresponds to the third reference voltage.

According to this modification, compared with the configurationillustrated in FIG. 24, the noise suppression ratio can be significantlyimproved by the effects of the third capacitor 19 and the fourthcapacitor 20.

Since the two capacitances are arranged, the noise suppression effectincreases. However, the arrangement area also increases. Since thesuppression effect varies depending on the presence/absence of acapacitor and the absolute value of a capacitance, the designer canperform design through selection of an arbitrary configuration andvalues.

Fifth Embodiment

The structure, functions, and a drive method of an imaging device 100according to a fifth embodiment will be described with reference toFIGS. 28 to 34. The imaging device 100 according to the presentembodiment differs from the imaging device 100 according to the fourthembodiment in that a switching unit 4B is added to the reading circuit50D according to the fourth embodiment. The following description willbe mainly given of differences from the fourth embodiment.

FIGS. 28 and 29 each schematically illustrate an exemplary circuitconfiguration of each unit pixel cell 110E in the imaging device 100according to the present embodiment. Each unit pixel cell 110E includesa photodetector 1 and a reading circuit 50E. The reading circuit 50Eincludes an amplifier 2B, an FD, a switching unit 4B, and an outputselector 5B.

The structure and functions of the reading circuit 50E will be describedwith reference to FIG. 30.

FIG. 30 schematically illustrates one example of the circuitconfiguration of the reading circuit 50E. The switching unit 4B includesa switch transistor 401. A fifth control signal line CON5 is connectedto the gate of the switch transistor 401. The FD is connected to one ofthe source and the drain of the switch transistor 401. The fourthreference voltage VR4 is connected to the other of the source and thedrain of the switch transistor 401. The sixth control signal line CON6is connected to one of the source and the drain of the thirdamplification transistor 202.

Next, an operation flow of the reading circuit 50E will be describedwith reference to FIG. 31.

FIG. 31 is a timing chart illustrating one example of the operation ofthe reading circuit 50E. The horizontal axis in each graph representstime, and the vertical axes represent a voltage of the fifth controlsignal line CON5, a voltage of the sixth control signal line CON6, and avoltage of the seventh control signal line CON7 sequentially from thetop in FIG. 31.

(Reset Period)

At time t28, the voltage of the fifth control signal line CON5 is set tothe high level to turn on the switch transistor 401. At this point intime, the fourth reference voltage VR4 and the FD are connected to eachother. Also, at time t28, the voltage of the seventh control signal lineCON7 is at the low level, and the second selection transistor 501 is inthe off state. That is, the fourth amplification transistor 203 iselectrically isolated from the signal reading line 7. In this state, thesixth control signal line CON6 is set to have the first referencevoltage so that the FD has a voltage that is equal or close to a desiredreset voltage VRST (=VR4). In this case, when the band of the thirdamplification transistor 202 is set to the third band, which is awideband, the FD, the gate of the third amplification transistor 202,and the other of the source and the drain of the third amplificationtransistor 202 are set to have a desired voltage at high speed.

As the voltage of the FD gets closer to the reset voltage VRST, theamount of time taken for the noise suppression becomes smaller, thusmaking it possible to reduce the drive time. Thus, it is desirable thata voltage be applied to the sixth control signal line CON6 so that thevoltage of the FD is equal or close to the reset voltage VRST. However,if the drive allows extra time, the setting value of the voltage is notlimited to that value.

At time t29, the voltage of the fifth control signal line CON5 is set tothe low level to turn off the switch transistor 401 so as to cause thefourth reference voltage VR4 and the FD to be disconnected from eachother.

(Noise Suppression Period)

In the state in which the switch transistor 401 is turned off and thefourth reference voltage VR4 and the FD are disconnected from eachother, the noise suppression operation and an operation for reading thesignal level or the reset level are performed.

In the period of time t29 to time t31, the seventh control signal lineCON7 remains at the low level, and the second selection transistor 501has been turned off. That is, the signal reading line 7 and the fourthamplification transistor 203 remain electrically isolated from eachother. In this state, the voltage of the sixth control signal line CON6is set to the second reference voltage. As a result, the state of thethird amplification transistor 202 changes gradually from the on stateto the off state.

In the period of time t29 to time t31, the second reference voltage isset so that the band of the third amplification transistor 202 is thefourth band, which is narrower than the third band. This allows the bandof the third amplification transistor 202 to be limited to the band ofthe amplifier 2B formed by the feedback loop involving the thirdamplification transistor 202. In addition, the reset noise generated inthe third amplification transistor 202 can be efficiency suppressed inall bands.

After the noise is sufficiently reduced, at time t31, the voltage of thesixth control signal line CON6 is changed to the fourth referencevoltage with which the third amplification transistor 202 is completelyturned off. The feedback loop involving the third amplificationtransistor 202 is disconnected, and the voltage of the FD is stabilizedin a state in which the noise is suppressed.

The taper reset described above with reference to FIGS. 8 and 17 mayalso be applied in the noise suppression period in the presentembodiment. FIG. 32 is a timing chart illustrating one example of theoperation of the reading circuit 50E when the taper reset is applied. Asillustrated in FIG. 32, in the period of time t29 to time t30, thevoltage of the sixth control signal line CON6 may be gradually changedso as to cross the threshold voltage in the range of the secondreference voltage to the third reference voltage. The state of the thirdamplification transistor 202 changes gradually from the on state to theoff state. As a result, noise generated in the FD can be suppressed inall bands.

(Exposure/Reading Period)

In a desired period when noise in the FD is sufficiently suppressed andthe voltage is stable, signal charge is accumulated in the FD.Thereafter, at time t32, the second selection transistor 501 is turnedon, so that the fourth amplification transistor 203 is electricallyconnected to the signal reading line 7. Thus, the fourth amplificationtransistor 203 and the constant current source 6 form a source followercircuit. The signal charge accumulated in the FD is amplified by thesource follower circuit, and the amplified signal is output to theperipheral circuitry (such as a CDS circuit and an A/D circuit) throughthe signal reading line 7.

In the present embodiment, controlling the switch transistor 401 makesit easy to set the FD to have a desired reset voltage VRST at highspeed.

In the fourth embodiment, the gain of the amplifier 2A is −A, and resetnoise generated by the second amplification transistor 201 or the thirdamplification transistor 202 is fed back while band limitation isapplied. As a result, the reset noise is reduced to 1/(1+A)^(1/2).

In contrast, according to the present embodiment, since the feedback isperformed after the switch transistor 401 is turned off, the reset noisegenerated by the switch transistor 401 can be significantly reduced to1/(1+A)^(1/2). The reset noise generated by the third amplificationtransistor 202 is also reduced to 1/(1+A)^(1/2) by performing thefeedback while applying the band limitation. In addition, as in themodification in the fourth embodiment, when the capacitance C3 of thethird capacitor 19 is set to a value larger than the capacitance Cfd ofthe FD, kTC noise generated by the third amplification transistor 202can be reduced to (kT/C3)^(1/2)<(kT/Cfd)^(1/2). In addition, when thefourth capacitor 20 is set to be smaller than the capacitance Cfd in theFD, the amount of noise in the FD can be attenuated to C4/(Cfd+C4) bydivided voltages of the capacitance Cfd of the FD and the capacitance C4of the fourth capacitor 20.

An advantage offered by the present embodiment is specifically comparedwith an advantage offered by the configuration described above in thefourth embodiment and illustrated in FIGS. 24 and 27. According to theconfiguration illustrated in FIG. 24, when the gain of the amplifier 2Ais represented by A, and the gain of the second amplification transistor201 is represented by A′, the reset noise in the second amplificationtransistor 201 is reduced to 1/(1+A)^(1/2)=1/(1+A′)^(1/2). In contrast,according to the configuration illustrated in FIG. 27, when the gain ofthe amplifier 2A is represented by A, and the gain of the thirdamplification transistor 202 is represented by A′, the reset noise inthe third amplification transistor 202 is reduced to1/(1+A)^(1/2)=1/[1+A′×{C4/(Cfd+C4)}×(C3/Cfd)]^(1/2). Thus, the resetnoise can be suppressed compared with the configuration illustrated inFIG. 24.

On the other hand, according to the present embodiment, when the gain ofthe amplifier 2B is represented by A, and the gain of the thirdamplification transistor 202 is represented by A′, the reset noise inthe switch transistor 401 is reduced to 1/(1+A)=1/[1+A′×{C4/(Cfd+C4)}].The reset noise in the third amplification transistor 202 is alsoreduced to 1/(1+A)^(1/2)=1/[1+A′×{C4/(Cfd+C4)}×(C3/Cfd)]^(1/2). Sincethe total noise can be obtained from a square root of the sum of squaresof these noise components, the reset noise can be significantly reducedcompared with the fourth embodiment.

According to the present embodiment, compared with the fourthembodiment, the reset noise can be significantly reduced by the effectsof the third capacitor 19 and the fourth capacitor 20. Also, theprovision of the switching unit 4B makes it easier to perform resettingand noise suppression at high speed.

As described above, the provision of the third capacitor 19, the fourthcapacitor 20, and the switching unit 4B provides a greater noisesuppression effect. However, the arrangement area also increases. Sincethe noise suppression effect depends on the presence/absence of acapacitor and the absolute value of a capacitance, the designer canperform design through selection an arbitrary configuration and theabsolute value of a capacitance.

The following description will be given of a modification of the readingcircuit 50E according to the present embodiment.

FIGS. 33 and 34 schematically illustrate other exemplary circuitconfigurations of the reading circuit 50E. As illustrated in FIG. 33,one of the source and the drain of the switch transistor 401 may beconnected to the FD, and the other of the source and the drain of theswitch transistor 401 may be connected to the sixth control signal lineCONE. With this configuration, it is possible to execute resettingwithout applying the fourth reference voltage VR4 and it is possible toprovide advantages that are similar to those in the configurationillustrated in FIG. 30.

Also, as another modification of the switching unit 4B, one of thesource and the drain of the switch transistor 401 may be connected tothe FD, and the other of the source and the drain of the switchtransistor 401 may be connected to a node (RD) of the third capacitor 19and the fourth capacitor 20, as illustrated in FIG. 34. With thisconfiguration, it is possible to execute resetting without applying thefourth reference voltage VR4, and it is possible to provide advantagesthat are similar to those in the configuration illustrated in FIG. 30.According to this configuration, in particular, since the gate of thethird amplification transistor 202 and the other of the source and thedrain of the third amplification transistor 202 can be set to have thesame voltage, the time of the noise cancellation can be reduced.

Although the operation of the feedback circuit 30 or 30′ involving thenegative feedback has been described above in the first to fifthembodiments, the feedback is not limited thereto. Positive feedback mayalso be added to the feedback. For example, after positive feedback isapplied, negative feedback may be applied to suppress noise, or thefeedbacks may be performed in the reverse order thereof to suppressnoise. Also, noise may be suppressed while applying positive feedbackand negative feedback at the same time. Use of the positive feedback inconjunction with the negative feedback makes it possible to increase thespeed and the efficiency of the noise suppression.

Sixth Embodiment

A camera system 600 according to a sixth embodiment will be describedwith reference to FIG. 35.

FIG. 35 schematically illustrates an example of the configuration of thecamera system 600 according to the present embodiment. The camera system600 has a lens optical system 601, an imaging device 602, a systemcontroller 603, and a camera-signal processor 604.

The lens optical system 601 includes, for example, a lens for autofocus,a lens for zoom, and a diaphragm. The lens optical system 601 condenseslight on an image capture plane of the imaging device 602. The imagingdevice described above in each of the first to fifth embodiments can bewidely used as the imaging device 602.

The system controller 603 controls the entire camera system 600. Thesystem controller 603 is implemented by, for example, a microcomputer.

The camera-signal processor 604 functions as a signal processing circuitfor processing a signal output from the imaging device 602. Thecamera-signal processor 604 performs processing, for example, gammacorrection, color interpolation processing, space interpolationprocessing, and automatic white balancing. The camera-signal processor604 can be implemented by, for example, a digital signal processor(DSP).

According to the camera system in the present embodiment, when theimaging device according to any of the first to fifth embodiments isused, it is possible to appropriately suppress reset noise (kTC noise)during reading. As a result, charge can be correctly read, and a goodimage can be obtained.

The imaging device according to the present disclosure can be applied tovarious sensor systems and camera systems, such as digital stillcameras, medical cameras, cameras for monitoring, vehicle-mountedcameras, digital single-lens reflex cameras, and digital mirrorlessinterchangeable lens cameras.

What is claimed is:
 1. An imaging device comprising a unit pixel cellcomprising: a photoelectric converter that generates an electric signalthrough photoelectric conversion of incident light; and a signaldetection circuit that detects the electric signal, the signal detectioncircuit comprising a first transistor that amplifies the electricsignal, a second transistor that selectively transmits output of thefirst transistor to outside of the unit pixel cell, and a feedbackcircuit that forms a feedback loop through which the electric signal isnegatively fed back, the feedback loop not passing through the firsttransistor.
 2. The imaging device according to claim 1, wherein thefirst transistor is provided out of the feedback loop.
 3. An imagingdevice comprising a unit pixel cell comprising: a photoelectricconverter that generates an electric signal through photoelectricconversion of incident light; and a signal detection circuit thatdetects the electric signal, the signal detection circuit comprising afirst transistor that amplifies the electric signal, a second transistorthat selectively transmits output of the first transistor to outside ofthe unit pixel cell, and a feedback circuit that forms a feedback loopthrough which the electric signal is negatively fed back, the feedbackloop passing through the first transistor, the feedback circuitcomprising a third transistor provided on the feedback loop, a gate ofthe third transistor being connected to a voltage control circuit thatoutputs at least three voltages different from each other.
 4. Theimaging device according to claim 3, wherein the first transistor isprovided on the feedback loop.
 5. The imaging device according to claim3, further comprising a constant current source provided on an outputside of the first transistor.
 6. The imaging device according to claim1, wherein the signal detection circuit further comprises a resettransistor that initializes the electric signal.
 7. The imaging deviceaccording to claim 1, wherein the signal detection circuit amplifies theelectric signal with a first amplification factor in a period when thefeedback circuit forms the feedback loop, and the signal detectioncircuit amplifies the electric signal with a second amplification factordifferent from the first amplification factor in a period when thefeedback circuit does not form the feedback loop.
 8. The imaging deviceaccording to claim 6, wherein the reset transistor is provided on thefeedback loop.
 9. The imaging device according to claim 8, wherein thereset transistor functions as a negative feedback transistor.
 10. Theimaging device according to claim 1, wherein the feedback circuitcomprises a first capacitor and a second capacitor connected in seriesto the first capacitor, and the first capacitor and the second capacitorare provided between the photoelectric converter and a referencevoltage.
 11. The imaging device according to claim 10, wherein the firstcapacitor is provided on the feedback loop.
 12. The imaging deviceaccording to claim 10, wherein the first capacitor has a firstcapacitance value, and the second capacitor has a second capacitancevalue larger than the first capacitance value.
 13. The imaging deviceaccording to claim 1, wherein the feedback circuit comprises a thirdtransistor having a gate, a source and a drain, the gate and one of thesource and the drain being connected to the photoelectric converter, andthe feedback loop comprises a path passing through the third transistorfrom the gate to the one of the source and the drain.
 14. The imagingdevice according to claim 10, wherein the feedback circuit furthercomprises a third transistor having a gate, a source and a drain, thegate being connected to the photoelectric converter, one of the sourceand the drain being connected to the photoelectric converter via thefirst capacitor, and the feedback loop comprises a path passing throughthe third transistor from the gate to the one of the source and thedrain and passing through the first capacitor.